Direct Frequency Response Analysis of a Flat Plate - OS-1100



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This tutorial demonstrates how to import an existing FE model of a flat plate, apply boundary conditions, and perform a finite element analysis of the problem. The flat plate will be subjected to a frequency-varying unit load excitation using the direct method. Post-processing is done in HyperView and HyperGraph to visualize deformations, mode shape response, and frequency-phase output characteristics.

The following exercises are included:

·     Setting up the problem in HyperMesh

·     Submitting the job

·     Viewing the results (HyperMesh and HyperGraph)

The following file is needed to perform this tutorial:

direct_response_flat_plate_input.fem  Original ASCII OptiStruct input deck

This file can be found in <install_directory>/tutorials/os/ and copied to your working directory.

To load the OptiStruct user profile and retrieve an existing FE (OptiStruct) model:

  1. Launch HyperMesh.

  2. Choose the OptiStruct in the User Profile dialog and click OK.

  3. Select the Files panel toolbar button image\files_panel.gif.

  4. Select the import subpanel using the radio button on the left of the panel.

  5. Select FE.

  6. Click import….

An Open file… browser window pops up.

  1. Select the direct_response_flat_plate_input.fem file, located in <install_directory>/tutorials/os/.

  2. Click Open.

The direct_response_flat_plate_input.fem model loads into the current HyperMesh session.

  1. Click return to get back to the main menu.

To apply loads and boundary conditions to the model:

In this section, the model is constrained at one edge. A unit vertical load is applied acting upwards in the positive z-direction at a point on a free edge corner of the plate.

To create two load collectors, spcs and unit-load:

  1. Select the Collectors toolbar button image\collectors.gif.

  2. Select the create subpanel using the radio buttons on the left-hand side of the panel.

  3. Click the type switch and select load collectors from the pop-up menu.

  4. Click name = and enter spcs.

  5. Click color and select a color.

  6. Click the creation method switch and select no card image from the pop-up menu.

  7. Click create.

A new load collector, spcs, is created.

  1. Click name = and enter unit-load.

  2. Click color and select a color.

  3. Click create.

A new load collector, unit-load, is created.

  1. Click return to return to the main menu.

Next, we will set the current load collector.

  1. Make sure that the current load collector is set to spcs by clicking comp: in the message bar image\comp.gif.

This opens a menu that displays the collector currently selected for each type. (If not already selected, click loadcol here and choose spcs).

image\comps_spcs.gif

  1. Select the Tool page.

  2. Select numbers then click nodes and select displayed from the extended entity selection menu.

  3. Select on (green button).

All of the node numbers on the flat plate should now be displayed.

  1. Click return to get to the main menu.

To create constraints:

  1. From the Analysis page, select the constraints panel.

  2. Select the create subpanel using the radio buttons on the left-hand side of the panel.

  3. Click the entity selection switch and select nodes from the pop-up menu.

  4. Click nodes and select nodes 5, 29, 30, 31 and 32 (see figure).

image\os0008-fig1.gif

Illustration of which nodes to select for applying single point constraints

  1. Constrain dof1, dof2, dof3, dof4, and dof5.

Dofs with a check will be constrained while dofs without a check will be free.

Dofs 1, 2, and 3 are x, y, and z translation degrees of freedom.

Dofs 4, 5, and 6 are x, y, and z rotational degrees of freedom.

You will need only to un-check dof 6.

  1. Click create.

The selected nodes will be free to rotate about the z-axis since dof6 was not checked.

  1. Click return to return to the main menu.

To create a unit load at a point on the flat plate:

  1. Press G on the keyboard to access the Global panel.

  2. Set the current collectors: to unit-load (by clicking on loadcols= and selecting unit-load from the pop-up menu).

  3. Click return to return to the main menu.

  4. Select load types from Analysis page.

  5. Select constraint = and choose DAREA from the extended entity selection menu.

  6. Click return to exit the Load Types panel.

  7. Select the constraints panel on the Analysis page.

  8. Select the create subpanel using the radio buttons on the left-hand side of the panel.

  9. Click the entity selection switch and select nodes from the pop-up menu.

  10. Select node number 19 on the plate by clicking on it (see figure).

image\os0008-fig2.gif

Node selected for creating unit vertical load.

  1. Un-check all dofs except dof3.

  2. Click the = to the right of dof3, and type in a value of 1.

  3. Click create.

This applies a unit load to the selected node.

  1. Click return to exit the constraints panel.

To create a frequency range table:

  1. Select the Collectors toolbar button image\collectors.gif.

  2. Select the create subpanel using the radio buttons on the left-hand side of the panel.

  3. Click the collector type switch and select load collectors from the extended entity selection menu.

  4. Click name = and enter tabled1.

  5. Click color and select a color.

  6. Click the creation method switch and from the pop-up menu select card image.

  7. Click card image= and choose TABLED1 from the extended entity selection menu.

  8. Click create edit.

A new window appears in the work area screen.

  1. Click TABLED1_NUM = and input a value of 2.

  2. Leave the input field below x(1) set to 0.0.

  3. Click in the input field below y(1) a value 1.0.

  4. Click in the input field below x(2) a value 1000.0.

  5. Click in the input field below y(2) a value 1.0.

  6. Click return.

This gives a frequency range of 0.0 to 1000.0 with a constant 1.0 over this range.

  1. Click return to exit the collectors menu.

To create a frequency dependent dynamic load:

  1. Select the Collectors toolbar button image\collectors.gif.

  2. Select the create subpanel using the radio buttons on the left-hand side of the panel.

  3. Click the collector type switch and select load collectors from the extended entity selection menu.

  4. Click name = and enter rload2.

  5. Click color and select a color.

  6. Click the creation method switch and select card image from the pop-up menu.

  7. Click card image= and choose RLOAD2 from the pop-up extended entity selection menu.

  8. Click create/edit.

A new window appears in the work area screen.

  1. Double click on EXCITEID in the yellow box.

A list of collectors appears on the left-hand side.

  1. Select collector unit-load.

The ID 2 appears below the EXCITEID yellow box, this is the ID of the load collector unit-load.

  1. Double click on TB in the yellow box.

A list of collectors appears in the left-hand, bottom corner.

  1. Select collector tabled1 (last on the list of collectors).

  2. Click return to exit the Collectors menu.

The type of excitation can be an applied load (force or moment), an enforced displacement, velocity or acceleration. The field [type] in the ROLOAD2 card image defines the type of load. The type is set to applied load by default.

To create a set of frequencies to be used in the response solution:

  1. Select the Collectors toolbar button image\collectors.gif.

  2. Select the create subpanel using the radio buttons on the left-hand side of the panel.

  3. Click the collector type switch and select load collectors from the extended entity selection menu.

  4. Click name = and enter freq1.

  5. Click color and select a color.

  6. Click the creation method switch and choose card image from the pop-up menu.

  7. Click card image= and choose FREQ1 from the extended entity selection menu.

  8. Click create edit.

A new window appears showing the card image of FREQ1.

  1. Click F1, then click in the field box below it and input a value of 20.0.

  2. Next, click DF, then click in the field box below it and input a value of 20.0.

  3. Next, click NDF, then click in the field box below it and input a value of 49.

  4. Click return.

This gives you a set of frequencies beginning with 20.0, incremented by 20.0 and 49 frequencies increments.

  1. Click return to exit the Collectors menu.

To create an OptiStruct subcase (also referred to as a loadstep):

  1. Select the subcase panel on the Analysis page.

  2. Click the type: switch and choose freq.resp (direct) from the pop-up menu.

  3. Click name = and enter subcase1.

  4. Check the box preceding SPC.

An entry field appears to the right of SPC.

  1. Click on the entry field and select spcs from the list of load collectors.

  2. Check the box preceding DLOAD.

An entry field appears to the right of DLOAD.

  1. Click on the entry field and select rload2 from the list of load collectors.

  2. Check the box preceding FREQ.

An entry field appears to the right of FREQ.

  1. Click on the entry field and select freq1 from the list of load collectors.

  2. Click create.

An OptiStruct subcase has been created which references the constraints in the load collector spc and the unit load in the load collector rload2 with a set of frequencies defined in load collector freq1.

  1. Click return to go to the main menu.

To create a set of nodes for output of results:

  1. From the Analysis page, select the entity sets panel.

  2. Click on name = and type in SETA.

  3. Leave the Set type: switch set to non-ordered type.

  4. Click the switch below name and choose no card instead of card image.

  5. Make sure that the yellow entity: selection type box is set to nodes.

  6. Select nodes with IDs 15, 17 and 19.

  7. Click create.

A message appears stating The entity set has been created.

  1. Click return to exit the entity sets menu.

Create a set of outputs and mass factors specific to Frequency Response analysis:

  1. From the Analysis page, select the control cards panel.

  2. Select DISPLACEMENTS.

A new window appears in the work area screen.

  1. Click the DISP_FORM box and select PHASE from the pop-up menu.

  2. Click the DISP_OPT field box and select SID from the pop-up menu.

A new field appears in yellow.

  1. Double click the SID box and select SETA.

A value of 1 now appears below the SID field box.

This sets the output for only the nodes in set 1.

  1. Click return to exit the DISPLACEMENTS menu.

  2. From the Control Cards panel, select FORMAT.

A new window appears in the work area screen.

  1. Click number_of_formats = , and input a value of 2.

  2. On the extended menu in the work area, click on the first FORMAT_V1 field box and select OPTI from the pop-up menu.

Using OPTI would generate OptiStruct ASCII result files like .disp, .strs, etc. as the output once the run is complete. These files are used during post-processing.

  1. Make sure the second field box is set to H3D.

  2. Click return to exit the FORMAT menu and return to the Control Cards menu.

  3. Click next twice and select the PARAM subpanel.

  4. Scroll down the list using the arrow in the left corner and check the box next to COUPMASS.

A new PARAM card appears in the work area screen.

  1. Click NO below COUPM_V1 and select 1 from the pop-up menu selection.

Selecting 1 uses the coupled mass matrix approach for eigenvalue analysis.

  1. Check the box next to G.

A new PARAM card appears in the work area screen.

  1. Click below G_V1, and input a value of 0.06 into the field box.

This value specifies a uniform structural damping coefficient and is obtained by multiplying the critical damping [image\cco2.gif] ratio by 2.0.

  1. Scroll down using the arrow in the left corner and check the box next to WTMASS.

A new window appears in the work area screen.

  1. Click below WTM_V1, and input a value of 0.102 into the field box.

Three PARAM statements should now appear in the pop-up menu on the work screen.

This factor is used to input all mass entries in weight units. Using this param multiplies all terms in the mass matrix by this factor.

  1. Click return to exit the PARAM menu.

  2. Click prev to move to the last page.

A new window appears in the work area.

  1. Verify that KEYWORD is set to HGFREQ.

Using HGFREQ will result in a frequency output presentation for HyperGraph.

  1. Double click on the box beneath FREQ and select ALL from the pop-up selection.

Choosing ALL will output results for all frequencies.

  1. Leave number_of_outputs set equal to 1.

  2. Click return to exit OUTPUT.

  3. Click return to exit the control cards menu.

Submitting the Job

To launch OptiStruct:

  1. Select OptiStruct from the Applications pull-down menu.

  2. Following the input file: field, click save as….

  3. Select the directory where you would like to write the OptiStruct model, enter the name flat_plate_direct_response.fem for the model in the File name: field, and click Save.

  4. Select run options: analysis.

  5. Click OptiStruct.

This launches the OptiStruct job. If the job is successful, new result files can be seen in the director where the OptiStruct model file was written. The flat_plate_direct_response.out file is a good place to look for error messages that will help to debug the input deck if any errors are present.

The default files written to the directory are:

flat_plate_direct_response.html

HTML report of the analysis, giving a summary of the problem formulation and the analysis results.

flat_plate_direct_response.out

OptiStruct output file containing specific information on the file set up, the set up of your optimization problem, estimates for the amount of RAM and disk space required for the run, information for each optimization iteration, and compute time information. Review this file for warnings and errors.

flat_plate_direct_response.h3d

HyperView compressed binary results file.

flat_plate_direct_response.stat

Summary of analysis process, providing CPU information for each step during the analysis process.

Viewing the Results

This section describes how to view displacement results (.mvw file) in HyperGraph and also explains the displacement output (.disp file) from this run. The HyperView results file (.h3d) contains only the displacement results for the three nodes specified in the node set output.

  1. Click the HyperView button to launch HyperView.

image\hv.gif

  1. Click Close to close the Message Log window.

  2. In the HyperView window, select the File pull-down menu from the toolbar and select Open….

The Open Session File window is displayed.

  1. Select the directory where the job was run and choose the file flat_plate_direct_response_freq.mvw.

  2. Click Open.

  3. A warning appears asking whether to discard the existing contents. Click Yes.

Two graphs per page, and a total of three pages, are displayed.

The graph title shows Subcase 1 Displacements of grid 15 on page 1.

There are two sets of results on this page. The top graph shows Phase Angle verses Frequency (log). The bottom graph shows Magnitude versus Frequency (log) (see figure) for Displacements at grid 15. .

image\os0008-fig3-new.gif

Frequency response of node 15.

  1. Directly underneath the blue graph border, select the right arrow button.

This displayed page 2, which shows Subcase 1 Displacements of grid 17 (see figure).

image\os0008-fig4-new.gif

Frequency response of node 17.

  1. Select the right arrow button again to display page 3 containing Subcase 1 Displacements of grid 19 (see figure).

image\os0008-fig5-new.gif

Frequency response of node 19.

This concludes the HyperGraph results processing.

  1. Open the displacement file (.disp) using a text editor.

The first field on the second line shows the iteration number, the second field shows the number of data points, and the third field shows the iteration frequency.

Line 3, first field shows node number, then x, y, and z displacement magnitudes and x, y,, and z rotation magnitudes.

Line 4, first field shows node number, then x, y, and z displacement phase angles and x, y, and z rotation angles.

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